The present invention relates to organic electroluminescent (EL) devices, also known as organic light-emitting diodes (OLED), and particularly to apparatus, which facilitates forming organic layers in such devices.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium may consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium may provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electron transport layer (for electron transport). In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, there are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision (hole position of xc2x15 micrometers) for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. Such a method is disclosed by Littman in U.S. Pat. No. 5,688,551, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553).
In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, and blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material can be precoated on one surface of a donor support material and transferred to a substrate by vapor deposition in a selected pattern (as in FIGS. 4, 5, and 6 in the aforementioned U.S. Pat. No. 5,937,272).
The EL material transfer is preferably done in a vacuum chamber such as Tang describes in the aforementioned patent and, in particular, vacuum is preferably maintained between the donor and substrate. The donor and substrate must also be kept in close proximity during the EL transfer (less than 250 micrometers between the coating and raised portions of the substrate as taught by Tang). Furthermore, the donor may be in contact with the raised portions of the substrate and thereby maintain sufficient spacing between the coating and the recessed portions of the substrate where the EL material is deposited. In any case, a method of holding the donor and substrate in contact in a vacuum chamber while maintaining vacuum between the donor and substrate is required.
Isberg et al., in commonly assigned European Patent Application 1 028 001 A1, have disclosed the additional use of an adhesion-promoting layer between the donor layer and substrate. While this would help promote the close contact required by Tang, it would be disadvantageous because the adhesion-promoting layer can introduce impurities in the form of the adhesive.
Mechanical pressure, such as that applied by a manual plate, can be used but is difficult to maintain evenly over the entire surface for the micrometer-order tolerances needed. Pressure from air or other fluids would work better, but the use of such pressure is made difficult in that the conditions in the vacuum chamber need to remain undisturbed.
It is therefore an object of the present invention to provide a more effective way of positioning a donor web to an OLED substrate for facilitating the formation of one or more layers of organic material.
This object is achieved by an apparatus for transferring organic material from a donor web onto a substrate to form a layer of organic material on one or more OLED devices, comprising:
a) a web of donor material;
b) a first fixture arranged to engage the donor web and substrate in a relationship relative to one another whereby there will be either a separation between portions of the substrate and the donor web, or the substrate and donor web will be in contact, and wherein organic material will be transferred onto portions of the substrate;
c) a second fixture aligned with and engaging the first fixture to clamp the donor web and substrate and forming a chamber relative to a non-transfer surface of the donor web;
d) means for moving the donor web to position a portion of the donor web in the chamber at an organic material transfer position;
e) means for supplying a fluid to the chamber to apply pressure to the non-transfer surface of the donor web so as to ensure the position of the donor web relative to the substrate;
f) the first fixture including a transparent portion located in relationship to the non-transfer surface of the donor web to permit transmission of radiation through such transparent portion to the non-transfer surface of the donor web so that heat will be produced and the organic material will transfer from the donor web to the substrate; and
g) means including a laser light source that is directed through the transparent portion to illuminate the donor web in a pattern which will cause the appropriate transfer of the organic material to the substrate.
An advantage to the present invention is that by using a web of donor material the productivity of manufacturing OLED devices is increased. The apparatus maintains a uniform spacing between a donor material and a substrate in an ambient vacuum or vacuum environment and where it is further preferred that vacuum be maintained between the donor and substrate. This provides for suitable clamping in an environment (vacuum) that is advantageous for lowering contamination. A further advantage is that this method can be fully automated including donor web and substrate media handling, providing for higher throughput. The present invention is particularly suitable for forming organic layers over a large area having a number of OLED display devices, which are in the process of being formed. A further advantage of this invention is that it allows for the manufacture of large amounts of organic donor material that can be stored until needed.